1. Field of the Invention
The present invention relates in general to a grindstone, such as a gear-tooth honing stone, having a resinoid abrasive structure including abrasive agglomerates each of which is provided by a vitrified abrasive structure.
2. Discussion of the Related Art
A gear-tooth honing operation is known as a hardened-gear finishing operation that is intended to reduce the noise of spur or helical gears (e.g., which are to be used in a vehicle transmission unit) after their heat treatment. That is, such a gear-tooth honing operation is effected for removing a distortion of the gear caused during the heat treatment, and for improving accuracy of each tooth of the gear. As a gear-tooth honing stone used in the gear-tooth honing operation, there is known a resinoid grindstone in the form of an internally-toothed honing wheel having an annular main body provided by an abrasive structure in which standard abrasive grains such as alumina oxide grains and silicon carbide grains are held together by a binder including a resin bond that is made of a thermosetting resin such as phenol and epoxy. The annular main body has, in its inner circumferential surface, a teeth profile portion which is to be meshed with external teeth of a gear as a workpiece. Such an internally-toothed honing wheel is subjected to a dressing operation with a dressing gear having a surface on which diamond abrasive grains are fixed in an electrodeposition method. In a honing operation executed after the dressing operation, the work gear is brought into engagement at its external teeth with the teeth profile portion of the internally-toothed honing wheel, and is rotated about its axis while being reciprocated in the axial direction, whereby the surface of each tooth of the work gear is ground or honed.
The resinoid grindstone is elastically deformed in a larger amount when a grinding load is applied thereto, than a vitrified grindstone. Further, the resinoid grindstone has a smaller degree of strength for holding or bonding the abrasive grains together, and a lower degree of wear resistance than the vitrified grindstone. Due to such characteristics thereof, the resinoid grindstone is not capable of satisfying a recent demand for grinding or removing a larger volume of stock from a workpiece with a higher efficiency, even where the abrasive structure of the resinoid grindstone is modified to have a higher density and a higher grade (that corresponds to the degree of strength with which the binder or bond holds the abrasive grains in the bond setting).
Where the resinoid grindstone takes the form of a honing stone (like the above-described internally-toothed honing wheel) having a teeth profile which is formed in its outer or inner circumferential surface and which conforms to teeth of a gear to be honed or ground as a workpiece, the honing stone has to be periodically or cyclically dressed so as to maintain its teeth profile so that the shape of the gear teeth finished by the honing stone is maintained. That is, when the teeth profile is changed to a certain extent due to wear of the honing stone, or when the amount of wear of the honing stone exceeds a predetermined amount, the honing stone has to be subjected to a dressing operation for rectifying or reshaping the teeth profile. The dressing cycle or interval is closely related with a service life of the honing stone. The wear resistance of the honing stone could be improved by using a resinoid grindstone of a high grade. However, such a resinoid grindstone having the high grade does not have a high degree of sharpness and is not accordingly capable of removing a large volume of stock from a workpiece with a high efficiency.
The present invention was made in the light of the background art discussed above. It is therefore an object of the present invention to provide a resinoid grindstone or a gear-tooth honing stone which has a high degree of wear resistance and is capable of removing a large amount of stock with a high efficiency. This object of the invention may be achieved according to any one of the first through fourteenth aspects of the invention which are described below.
The first aspect of this invention provides a resinoid grindstone comprising a main body having an abrasive structure in which abrasive agglomerates are held together by a binder including a resin bond, wherein each of the abrasive agglomerates includes abrasive grains which are held together by an inorganic bond, and wherein the binder further includes a filler such that a ratio of a strength of the abrasive agglomerates to a strength of the binder is not higher than 1.5.
In the resinoid grindstone according to this first aspect of the invention, each of the abrasive agglomerates included in the resinoid abrasive structure is provided by the plurality of abrasive grains held together by the inorganic bond, so that each abrasive agglomerate has a size larger than that of each of the abrasive grains included in each abrasive agglomerate. Each of the abrasive agglomerates, which are held by the resin binder, has a high rigidity owing to its vitrified abrasive structure. That is, the high rigidity of each abrasive agglomerate enables the present resinoid grindstone to have a large depth of cut with respect to a workpiece, while the resinoid grindstone as a whole has a certain degree of elasticity owing to the resinoid abrasive structure. Although each abrasive agglomerate is held together by the rein binder, each abrasive agglomerate having the relatively large size is supported at its relatively large area by the binder so that each abrasive agglomerate is not displaced by a large distance away from the workpiece during a grinding operation. Owing to this relatively large size of each abrasive agglomerate and presence of pores in the vitrified abrasive structure of each abrasive agglomerate, the resinoid grindstone is capable of removing a large amount of stock from the workpiece with a high efficiency. Further, owing to the relatively large size and also concavities and convexities in the surface of each abrasive agglomerate, each abrasive agglomerate is held together by the binder with a high bonding strength, thereby reducing removal of the abrasive agglomerates from the resinoid abrasive structure and accordingly improving the wear resistance.
Further, in the resinoid grindstone of this first aspect of the invention, the strength of the binder is increased by the incorporation of the filler in the binder such that the ratio of the strength of the abrasive agglomerates to the strength of the binder is not higher than 1.5. This arrangement makes it possible to improve a surface smoothness of the ground workpiece with a reduced risk of damaging or scratching the surface of the ground workpiece. Moreover, this arrangement serves to prolong the service life of a dressing tool which is used to dress the grindstone. It is noted that a content of the resin bond (included in the binder), a content of the abrasive agglomerates and a content of the filler (included in the binder) are preferably 15-30 wt %, 10-40 wt % and 40-75 wt %. This preferable proportions of the components are effective to further improve the grinding efficiency and the durability of the resinoid grindstone.
According to the second aspect of the invention, in the resinoid grindstone defined in the first aspect of the invention, the strength of the abrasive agglomerates is smaller than the strength of the binder. Namely, the ratio of the strength of the abrasive agglomerates to the strength of the binder is lower than 1.0., so that the surface smoothness of the ground workpiece is further improved with a further reduced risk of damaging or scratching the surface of the workpiece. Further, the service life of the dressing tool is further prolonged.
According to the third aspect of the invention, in the resinoid grindstone defined in the first or second aspect of the invention, the filler is provided by second abrasive grains each having a size which is not smaller than a size of each of the abrasive grains as first abrasive grains included in each of the abrasive agglomerates.
The fourth aspect of the invention provides a gear-tooth honing stone comprising a main body having a teeth profile portion to be meshed with a gear, for honing the gear, wherein the main body has an abrasive structure in which abrasive agglomerates are held together by a binder including a resin bond, wherein each of the abrasive agglomerates includes abrasive grains which are held together by an inorganic bond, and wherein the binder further includes a filler such that a ratio of a strength of the abrasive agglomerates to a strength of the binder is not higher than 1.5.
The gear-tooth honing stone of this fourth aspect of the invention provides substantially the same technical advantages as the grindstone of the first aspect of the invention. The advantage in prolonging the service life of the dressing tool is very significant, because a teeth-profiled dressing tool (dressing gear), which is expensive due to the electrodeposition of diamond abrasive grains on its surface, is used as the dressing tool to dress the gear-tooth honing stone.
According to the fifth aspect of the invention, in the gear-tooth honing stone defined in the fourth aspect of the invention, the strength of the abrasive agglomerates is smaller than the strength of the binder.
According to the sixth aspect of the invention, in the gear-tooth honing stone defined in the fourth or fifth aspect of the invention, the filler is provided by second abrasive grains each having a size which is not smaller than a size of each of the abrasive grains as first abrasive grains included in each of the abrasive agglomerates.
According to the seventh aspect of the invention, in the gear-tooth honing stone defined in the fourth through sixth aspects of the invention, the main body consists of an annular body, wherein the teeth profile portion is provided by a succession of protrusions formed in an inner circumferential surface of the annular body.
The eighth aspect of the invention provides a resinoid grindstone comprising a main body having an abrasive structure in which abrasive agglomerates are held together by a binder including a resin bond, wherein each of the abrasive agglomerates includes abrasive grains which are held together by an inorganic bond, and wherein a ratio of a total weight of the abrasive agglomerates included in the abrasive structure, to a weight of the abrasive structure is not lower than 10% and is not higher than 40%.
In the resinoid grindstone according to this eighth aspect of the invention in which a content of the abrasive agglomerates in the resinoid abrasive structure is 10-40 wt %, it is possible to enjoy the technical advantages provided by the abrasive agglomerates while preventing a reduction in a mechanical strength of the abrasive structure which would be caused by an excessively high content of the abrasive agglomerates. If the content of the abrasive agglomerates in the abrasive structure is lower than 10 wt %, an amount of the abrasive agglomerates exposed in a grinding surface of the grindstone is so small that the technical advantages of the abrasive agglomerates can not be practically enjoyed in a grinding operation. If the content of the abrasive agglomerates in the abrasive structure is higher than 40 wt %, a bonding strength of the abrasive structure of the grindstone is reduced to such an extent that makes it impossible to provide the abrasive structure of the grindstone with a required degree of mechanical strength, since the vitrified abrasive structure providing each abrasive agglomerate is more fragile or brittle than a resin structure. With the content of the abrasive agglomerates being within the above-described range 10-40 wt %, the reduction of the bonding strength of the grindstone is avoided whereby the required degree of mechanical strength is assured.
According to the ninth aspect of the invention, in the resinoid grindstone defined in the eighth aspect of the invention, the ratio of the total weight of the abrasive agglomerates included in the abrasive structure, to the weight of the abrasive structure is not higher than 30%.
The technical effect provided by the abrasive agglomerates generally increases with an increase in the content of the abrasive agglomerates in the resinoid abrasive structure. However, a further increase in the content of the abrasive agglomerates over 30 wt % no longer produces an appreciable increase in the technical effect. In this respect, it is preferable that the content of the abrasive agglomerates is not lower than 10 wt % and is not higher than 30 wt %, for thereby more reliably assuring the required degree of the mechanical strength of the main body of the grindstone, while maintaining a sufficiently high degree of machining ability of the grindstone.
According to the tenth aspect of the invention, in the resinoid grindstone defined in the eighth or ninth aspect of the invention, a ratio of a weight of the inorganic bond included in each of the abrasive agglomerates, to a weight of each of the abrasive agglomerates is not higher than 40%.
A suitable degree of hardness of each abrasive agglomerate is determined in view of a material of a workpiece to be ground and also a required degree of quality of the ground workpiece. In the resinoid grindstone of this tenth aspect of the invention in which the content of the inorganic bond in each abrasive agglomerate is not higher than 40 wt %, it is possible to prevent each of the abrasive agglomerates from having an excessively large hardness, and accordingly to avoid a deterioration in a surface smoothness of the ground workpiece. For obtaining a high degree of surface smoothness of the ground workpiece with minimized scratches on the ground surface, it is not desirable to excessively increase the hardness of each abrasive agglomerate that is dependent on the content of the inorganic bond in each abrasive agglomerate. It is preferable that the upper limit of the inorganic bond content is determined such that the mechanical strength of the abrasive agglomerates is lower than the mechanical strength of the binder, i.e., the mechanical strength of portions of the resinoid abrasive structure excluding the abrasive agglomerates.
According to the eleventh aspect of the invention, in the resinoid grindstone defined in the tenth aspect of the invention, the ratio of the weight of the inorganic bond included in each of the abrasive agglomerates, to the weight of each of the abrasive agglomerates is not lower than 5%.
An excessive reduction in the inorganic bond content leads to an easy destruction of each abrasive agglomerate. In this respect, if the content of the inorganic bond in each abrasive agglomerate is lower than 5 wt %, it is not possible to obtain a sufficiently high degree of durability of the grindstone in spite of the presence of the abrasive agglomerates in the abrasive structure of the grindstone. Therefore, the content of the inorganic bond in each abrasive agglomerate is preferably is at least 5 wt %, more preferably at least 10 wt %, for reliably obtaining the technical effect provided by the present of the abrasive agglomerates. Further, the inorganic bond content in each abrasive agglomerate is preferably not higher than 20 wt %, more preferably about 15 wt %, so that a sufficiently high degree of durability of the grindstone is obtained without suffering from a deterioration in the surface smoothness of the ground workpiece.
According to the twelfth aspect of the invention, in the resinoid grindstone defined in any one of the eighth through eleventh aspects of the invention, the binder includes second abrasive grains each having a size which is not smaller than a size of each of the abrasive grains as first abrasive grains included in each of the abrasive agglomerates, and which is smaller than a size of each of the abrasive agglomerates.
In the resinoid grindstone of this twelfth aspect of the invention in which the resinoid abrasive structure contains the multiplicity of second abrasive grains as well as the multiplicity of abrasive agglomerates, the second abrasive grains as well as the abrasive agglomerates are dispersed in the resin bond, the resinoid abrasive structure advantageously possesses further higher degrees of grade and structure density (grain percentage). Moreover, the arrangement, in which the grain size of the first abrasive grains constituting each abrasive agglomerate is smaller than the grain size of the second abrasive grains, is significant for improving surface smoothness of the ground workpiece. This is because the workpiece is ground mainly by the first abrasive grains rather than by the second abrasive grains, since the size of each of the abrasive agglomerates constituted by the first abrasive grains is larger than each of the second abrasive grains.
According to the thirteenth aspect of the invention, in the resinoid grindstone defined in any one of the eighth through twelfth aspects of the invention, the resinoid grindstone being a gear-tooth honing stone, wherein the main body has a teeth profile portion to be meshed with a gear, for honing the gear.
The principle of the invention is advantageously applied to a case where the resinoid grindstone is provided by the gear-tooth honing stone, as in this thirteenth aspect of the invention, which is commonly adapted to possess a high grade and a high degree of wear resistance required for grinding or honing the workpiece in the form of a gear having a surface hardened by a heat treatment. That is, in spite of the high grade and the high degree of wear resistance, the resinoid grindstone has a so-called spontaneous edge-forming capability or self-sharpening capability owing to the vitrified abrasive structure of each abrasive agglomerate in which the first abrasive grains are held together by the inorganic bond. The self-sharpening capability is a capability for restoring the sharpness of the grindstone from time to time by successive removals of the first abrasive grains from the abrasive agglomerate or small breakages of the first abrasive grains, and makes it possible to abrade the workpiece in such a manner that removes a large amount of stock with high efficiency and accuracy.
According to the fourteenth aspect of the invention, in the resinoid grindstone defined in the thirteenth aspect of the invention, the main body consists of an annular body, and wherein the teeth profile portion is provided by a succession of protrusions formed in an inner circumferential surface of the annular body.